and the Marsupials - A Historical and Modern Perspective
Marsupials became extinct
in North America by the end of the Oligocene, although they continued to
do well in South America, which by that time had become separated from
North America by an ocean channel (as a result of the ancient supercontinent
of Pangea breaking up). The mammalian
carnivore niches in South America were filled entirely by marsupials of
genera such as Borhyaena and Thylacosmilus,
the latter being an excellent example of a marsupial saber tooth "cat".
North America's present-day opossum (Didelphis virginiana) is not
descended from the continent's ancient marsupial stock, but is an immigrant
from further south that arrived during the Pleistocene Epoch. Even
though the geographical and chronological relationship between the marsupials
of Europe and the New World appears to now be fairly well understood,
the relationship of the Australian marsupials, which are today isolated
by wide ocean expanses from the other continents, was quite problematic
to explain prior to the discovery that the Earth's continents had drifted
over many millions of years.
skulls (both cast replicas) of Thylacosmilus, a remarkable marsupial
saber tooth predator from the Late Miocene and Early Pliocene of Argentina.
At left is Thylacosmilus atrox; at right, Thylacosmilus lentis.
It has been argued that lentis may
in fact be the same species as
atrox (Marshall, 1976).
| Because the only (currently)
convenient pathway to Australia is Asia, zoologists had long assumed that
the marsupial fauna of Australia had come from North America via Asia and
then "island-hopped" through Indonesia, probably during the Cretaceous
Period. For some mysterious reason the placentals failed to follow,
and the great southern land mass became a sanctuary for the marsupials,
which were free to evolve in isolation from them. A theory such as
this, however, has
problems. There is a distinct faunal break (Wallace's
Line) running along the deep channel between Java and Kalimantan
(Borneo) to the north-west (the Oriental realm) and Sulawesi (Celebes),
Irian (western New Guinea) and Australia (the Australasian realm) to the
south-east. Marsupials are present on many
of the islands that lie between Wallace's line and Australia, but because
there are far fewer species there than in Australia itself, it can be inferred
that colonization had been from the Australian mainland to the islands,
rather than vice versa. Placental mammals, however, are present only
on the western-most islands, with little overlap of the marsupial region.
This suggests that they spread from the Asian land mass.
Wallace line delineates the fauna of the Australian and Southeast Asian
fauna. It is named after naturalist Alfred Russel Wallace, who noticed
this clear division during his travels through the East Indies in the 19th
century. The probable extent of exposed land at the time of the last
glacial maximum, at which time the sea level was more than 110 meters lower
than is the case today, is shown in grey. The Lombok Strait between
Bali and Lombok formed a deep water barrier even when lower sea levels
linked the now-separated islands and landmasses on either side. Diagram
courtesy: Maximilian Dörrbecker.
Studies of continental drift have provided further support to suggest that
Asia was not the source of the Australian marsupial fauna. In
the mid-20th century, geological studies of plate tectonics showed that
there is an Indo-Australian plate that has Wallace's Line as a boundary,
resulting in a large drop in the sea floor at precisely the same point.
This means that it has never been possible for a land bridge to form in
the region, hence the zoological distribution.